1. Field of the Invention
The present invention relates to a color original readout apparatus which reads out the image of an original, as image data containing color data, with an image sensor such as a CCD or the like.
2. Description of the Prior Art
Image processing is known according to which the image of an original is read out as electric signals by an image sensor such as a CCD. According to the electric signals obtained, the image of the original is reproduced on a recording medium such as a paper sheet, is displayed on a display device such as a CRT or the like, or is transmitted to a remote place as in the case of a facsimile. Most of the image processing methods as described above process the image in monochrome. However, image processing of image data by readout of color data of a color original has been recently proposed.
In general color original readout apparatus, three image sensors (CCDs) are arranged with each having a filter of one of three colors red, green and blue to obtain signals for these three colors. On the basis of the signals obtained in this manner, an image is formed by a recording apparatus such as a laser beam printer, an ink jet printer or the like. In order to form the image with a laser beam printer, the part of the color image at which the black signal is obtained through the blue filter is reproduced with a yellow toner, the part of the color image which the black signal is obtained through the green filter is reproduced with a magenta toner, and the part of the color image at which the black signal is obtained through the red filter is reproduced with a cyan toner. In some apparatus, for the part of the image at which the black signal is obtained through all of these filters, printing with these toners is not performed and the part is reproduced in black by a black toner.
However, the color original readout apparatus of this type requires a plurality of image sensors. Since image sensor such as CCDs are expensive, this results in high manufacturing cost of the overall apparatus.
When a plurality of image sensors is used, a problem of mutual alignment of the sensors also occurs. If the positions of the image sensors deviate from one another, the image reproduced on the basis of the obtained image signals will have color drift. In order to prevent this, the readout positions of the image sensors must be carefully aligned. The size of the picture element widely adopted for a CCD as the image sensor is about 15.mu.. In order to solve problem as described above, the deviation from alignment of the image sensors must be corrected with a precision below several tenths the size of the picture element. This is almost impossible to accomplish.
In order to solve the problem of the mutual deviation of the image sensors and to lower the manufacturing cost of color original readout apparatus, it is proposed to obtain data corresponding to a plurality of colors with a single image sensor. It is proposed, for example, to rotate at high speed a plurality of filters of different colors in front of the image sensor to thereby color-separate the light which becomes incident on the image sensor. With this arrangement, the deviation in the direction of main scanning is eliminated. However, the readout positions on the original by subscanning differ slightly according to each filter. When an original is read out by separating the image in three colors, the image data for each color is obtained by each scanning operation. If the image data for the same color is read out upon displacement by one picture element in the direction of subscanning, the three image signals obtained upon color separation are obtained from positions which deviate from each other by 1/3 picture element. It is not desirable to reproduce the image from the three image signals which are read out from different positions of the original.
Another method is proposed by the same applicant. According to this method, the image of a original is color-separated into red and black using white and red light. The logic operations are performed according to which black is printed if the black signal is obtained with both lights, red is printed if the black signal is obtained with white light and the white signal is obtained with the red light, and white is printed in other cases. In this manner, the red and black data of the original is discriminated. However, with this method, during readout of a completely black original, the white signal is obtained with the red light and the black signal is obtained with the white light at the edge of the black image due to deviation in the subscanning direction. This results in an incorrect discrimination of red data. Thus, the edge of the black image is printed in red, providing poor reproduction.